We have previously demonstrated that serotonin neurons in the median raphe nucleus (MRN) facilitate anxiety- and fear-related behavior evaluated by elevated plus maze and fear conditioning tests, respectively. However, we still cannot conclude that the neuronal activity of serotonin neurons in the MRN directly causes negative emotion because the time gap between the experimental manipulations and the evaluation of resulting behavioral changes is relatively long in these behavioral tests. In other words, learning or planning processes could affect performance in these tests. To address this issue, we optogenetically stimulated serotonin neurons in the MRN and evaluated quick responses to sucrose reward in mice. We delivered a sucrose reward from a blunt-tipped needle placed at a licking distance and measured mice licking behavior. The optogenetic activation of serotonin neurons in the MRN reduced the number of licks. Furthermore, to assess hedonic responses more directly, we intraorally infused a drop of sucrose solution and analyzed taste reactivity and facial expression. We found that activating serotonin neurons in the MRN significantly reduced the number of tongue reactions and sweet sucrose-driven facial expressions. These results suggest that increased activity of serotonin neurons in the MRN mediates aversion.